What is Caterpillar Fault Code CID958?

CID958 indicates a communication error or data link failure within the Caterpillar machine's Controller Area Network (CAN) system, specifically related to a missing or unresponsive electronic control module (ECM) on the network. This diagnostic trouble code (DTC) means that the primary machine controller cannot establish or maintain communication with one or more critical system controllers, such as the Engine ECM, Hydraulic Control Module, or Display Module.

This fault is particularly critical for Caterpillar excavators because modern machines rely on constant data exchange between multiple electronic controllers to manage engine performance, hydraulic functions, and operator interface systems. When CID958 is active, the machine may enter a derate mode or experience reduced functionality as a protective measure. For used excavators, this code often surfaces due to age-related electrical degradation, corrosion, or previous repair work that compromised wiring integrity.

Common Symptoms

Operators may experience the following when CID958 is active:

• Warning light illumination on the instrument cluster, often accompanied by a "Check Engine" or system fault indicator
• Loss of specific machine functions, such as hydraulic controls, monitoring displays, or engine power management
• Intermittent or complete failure of the operator display screen showing machine parameters
• Engine derate or reduced power output as the system enters a fail-safe mode
• Multiple fault codes appearing simultaneously, indicating widespread communication breakdown across the CAN network

Potential Causes

The most common technical reasons for CID958 in used Caterpillar excavators include:

• Damaged or corroded CAN bus wiring harness, particularly at connection points or areas subject to vibration and wear
• Failed electronic control module (ECM) that has stopped communicating on the network
• Loose, corroded, or damaged electrical connectors at controller mounting locations
• Broken or frayed wiring at known rub points near the engine, boom pivot areas, or through bulkhead passages
• Poor ground connections causing voltage irregularities affecting CAN communication
• Water intrusion into controller housings or connector bodies, common in older machines with compromised seals
• Incorrect voltage supply to controllers due to battery, alternator, or power distribution issues

How to Troubleshoot and Fix Code CID958

Step 1: Perform Initial Diagnostics with Caterpillar Electronic Technician (Cat ET)

Connect Cat ET diagnostic software to the machine's diagnostic port. Document all active and logged fault codes. Use the software's network diagnostics feature to identify which specific ECM or controller is not responding on the CAN bus network. This pinpoints whether the issue involves the engine controller, hydraulic module, or display system.

Step 2: Inspect Physical Wiring and Connectors

Locate the CAN bus harness routing between the non-responsive controller and the main machine controller. For used excavators, pay special attention to harness sections that pass through high-vibration areas, sharp edges, or near heat sources. Inspect for chafed insulation, broken wires, or exposed conductors. Check all related electrical connectors for corrosion (green/white deposits), bent pins, or moisture intrusion. Clean connectors with electrical contact cleaner and apply dielectric grease before reconnection.

Step 3: Test CAN Bus Integrity with Multimeter

Using a digital multimeter, measure CAN High and CAN Low wire resistance between the suspect controller and a known good connection point. The CAN bus should show approximately 60 ohms resistance between CAN High and CAN Low when properly terminated. Check for voltage levels on both CAN wires with the key on—typically around 2.5V on each line with small fluctuations during communication. Significant deviation indicates wiring faults or termination resistor failures.

Step 4: Verify Power Supply and Ground Connections

Confirm the affected controller receives proper supply voltage (typically 12-24V depending on machine specifications) and has a solid ground connection. Measure voltage drop across ground connections—readings above 0.1V indicate poor ground integrity. For used machines, remove ground connection points, clean mating surfaces to bare metal, and reinstall with star washers to ensure reliable contact.

Step 5: Controller Replacement and Network Relearning

If wiring and power supply tests pass, the controller itself may have failed—common in older excavators with high operating hours. Replace the faulty ECM or control module with a genuine Caterpillar or OEM-equivalent part. After installation, use Cat ET software to perform controller configuration and network relearning procedures, ensuring the new module is properly recognized and communicates on the CAN network.

Important for Used Excavators: Before replacing expensive controllers, thoroughly inspect for mechanical wear on harness routing clips and grommets, as these commonly fail over time allowing wire movement and eventual failure. Check previous repair history, as improper aftermarket wiring repairs often cause intermittent CID958 faults.

---

Disclaimer: This guide provides general troubleshooting information for Caterpillar CID958 fault codes. Always consult the specific service manual for your excavator model and consider engaging a certified Caterpillar technician for complex electrical diagnostics. Improper repairs to electronic control systems can cause additional damage or safety hazards.